Creating Oxygen Vacancies as a Novel Strategy To Form Tetrahedrally Coordinated Ti4+ in Fe/TiO2 Nanoparticles

The incorporation of Fe as a dopant in anatase TiO2 nanoparticles has been systematically investigated with the aim of changing the coordination geometry of Ti via the formation of oxygen vacancies. Although Fe3+ ions are present in the solution during growth of the nanoparticles, a high-temperature heat treatment is found to be necessary to incorporate Fe3+ as a substituent for Ti4+ in the bulk of the TiO2 nanoparticles. The Fe3+ acceptors are found to be charge-compensated by oxygen vacancies, up to dopant concentrations as high as 10%. The surprisingly high solubility of Fe is attributed to the very similar radii of Ti4+ and Fe3+ and to the energetically favorable Coulomb attraction between the negatively charged Fe acceptor and the positively charged oxygen vacancies. A combined EXAFS/XANES study reveals that part of the Ti4+ ions changes their coordination number from 6 to 4 at high oxygen vacancy concentrations. The deliberate use of oxygen vacancies to modify the coordination geometry of metal ions represents a new strategy that offers exciting possibilities to tune the selectivity of photocatalytically active metal oxide nanoparticles.